An optical fiber is an optical waveguide having a central core surrounded by an outer cladding. The refractive indices of the core and cladding are selected so that optical energy propagating in the optical fiber is well-guided by the fiber.
As is well known in the art, a single optical fiber may provide one or more propagation paths under certain conditions. These propagation paths are commonly referred to as the normal modes of a fiber, which may be conceptualized as independent optical paths through the fiber. Normal modes have unique electric field distribution patterns which remain unchanged, except for amplitude as the light propagates through the fiber. Additionally, each normal mode will propagate through the fiber at a unique propagation velocity.
The number of modes which may be supported by a particular optical fiber is determined by the wavelength of the light propagating therethrough. If the wavelength is greater than a "second-order mode cutoff" wavelength (i.e., the frequency of the light is less than a cutoff frequency), the fiber will support only a single mode. If the wavelength is less than cutoff (i.e., the frequency is greater than the cutoff frequency), the fiber will begin to support higher order modes. For wavelengths less than, but near cutoff, the fiber will support only the fundamental, or first-order mode, and the next, or second-order mode. As the wavelength is decreased, the fiber will support additional modes, for example, a third-order, fourth-order, etc.
Each of the normal modes (e.g., first-order, second-order, etc.) are orthogonal, that is, ordinarily, there is no coupling between the light in these modes. The orientation of the electric field vectors of the modes defines the polarization of the light in the mode, for example, linear vertical or linear horizontal. A more complete discussion of these modes, and their corresponding electric field patterns, will be provided below.
A number of devices have been constructed to utilize the orthogonality of the modes of an optical fiber to provide selective coupling between the modes. For example, co-pending U.S. Pat. No. 4,768,851, entitled "Fiber Optic Modal Coupler," assigned to the assignee of this invention, describes a device which couples optical energy from the first-order mode to the second-order mode, and vice versa. U.S. patent application Ser. No. 048,142, entitled "Fiber Optic Inter-Mode Coupling Single-Sideband Frequency Shifter," assigned to the assignee of this invention, discloses frequency shifters which couple optical energy from one propagation mode to another propagation mode while shifting the frequency of the optical energy. U.S. patent application Ser. No. 820,411, entitled "Fiber Optic Mode Selector," assigned to the assignee of the present invention, discloses a device which separates optical energy propagating in one of the first-order and second-order propagation modes from the other of the first-order and second-order propagation modes.
In the optical fiber manufacturing industry, glass fibers have generally been used for conducting light waves therein. Materials other than glass, however, have also been considered for optical fibers. In particular, single crystal optical fibers, meaning fibers grown from a single crystal and having definite crystal planes, show attractive properties which distinguish them from conventional glass fibers. For instance, Neodymium YAG (yttrium, aluminum garnet) (Nd:YAG) crystals can be formed into both rods and thin fibers. In both forms, the crystal material can amplify light and function as a laser. More recently, lithium niobate crystals have been used to manufacture lithium niobate single crystal fibers. The ferroelectric domain structures of small diameter LiNbO.sub.3 single crystal fibers have been investigated in an article by Luh, et al., "Ferroelectric Domain Structures in LiNbO.sub.3 Single Crystal Fibers", Journal of Crystal Growth, 78, 1986, pp. 135-143. These structures were found to be different from those usually observed in large LiNbO.sub.3 crystals. Lithium niobate single crystal fibers have been manufactured using the laser-heated pedestal growth (LHPG) method, a variant of the float zone process. In such a method, the upper end of a source rod of the LiNbO.sub.3 crystal material is heated with a focused laser beam. A more detailed description of the methods used for producing such lithium niobate crystal fibers will be provided hereinbelow. More recently, a method for cladding lithium niobate crystal fibers has been described in "MgO:LiNbO.sub.3 single crystal fiber with magnesium-ion in-diffused cladding," Optics Letters, Vol 12, p. 938, November 1987. The methods for making such claddings for grown single crystal fibers were also disclosed in the aforementioned patent application. This application, U.S. patent application Ser. No. 186,045 filed Apr. 25, 1988 and assigned to Stanford University, is hereby incorporated herein by reference.
The present invention describes new all-fiber electro-optic modulators and other optical devices such as optical switches, of particular importance for modern optical communications and signal processing technologies. Integrated optic circuits, presently used to perform optical modulation and switching, do not exhibit the round geometry of glass fibers and, as a consequence, coupling losses are very high (the best commercially available integrated optic circuits have an insertion loss of 6 dB or higher). Very important fiber optic devices, such as electro-optic fiber modulators/switches and second harmonic generators, cannot be made of glass fibers because of the centrosymmetric properties of glass. As explained in more detail below, the second order nonlinear susceptibility vanishes for glass and makes glass unsuitable for electro-optic applications. On the other hand, single-crystal fibers are naturally compatible with glass fibers, because of their round or elliptical geometry and exhibit excellent electro-optic properties.